It is a well-known fact that an organopolysiloxane turns into a silicone rubber excelling in heat resistance, cold resistance, weather resistance, and electrical properties by heating in the presence of an organic peroxide or by hydrosilylation reaction. It is also known well that the silicone rubber can be obtained as the result of curing at room temperature by condensation reaction.
On the other hand, there is known an organopolysiloxane that cures upon irradiation with light in the presence of a photoinitiator.
A UV (ultraviolet)-curable organopolysiloxane composition is disclosed in JP-B S52-40334 and JP-A S60-104158 (Patent Documents 1 and 2). The composition includes a vinyl group-containing polysiloxane and a mercapto group-containing polysiloxane and forms a cured product through photo-radical addition reaction. Another example is disclosed in JP-B S53-36515, JP-A S60-215009, and JP 2647285 (Patent Documents 3 to 5). It is a photocurable organopolysiloxane composition including an acrylic group-containing polysiloxane and a sensitizer and forms a cured product upon irradiation with light.
Unfortunately, the above-mentioned composition containing a vinyl group-containing polysiloxane and a mercapto group-containing polysiloxane involves a problem with odor due to mercapto groups and corrosion on metal. Consequently, they are limited in applications.
The above-mentioned composition containing an acrylic group-containing polysiloxane and a sensitizer also suffers the disadvantage of requiring a high-molecular-weight linear polysiloxane as the acrylic group-containing polysiloxane for production of a rubbery elastic product. The higher the molecular weight the linear polysiloxane has, the less becomes the amount of the terminal acrylic group. This leads to poor curability. Another disadvantage is that the surface of the composition that is in contact with air cures very little due to inhibitive action by air. Thus, the compositions of this kind are not in practical use except for the one which contains a polysiloxane with a comparatively large content of acrylic groups and gives rise to a resinous cured product. None of them practically give rise to rubbery elastic products.
Moreover, the mercapto group-containing polysiloxane and the acrylic group-containing polysiloxane need a special synthetic route for their production. The resulting products are usually regarded as new chemical substances which are subject to legal regulations and disadvantageous costwise.
The photocurable organopolysiloxane composition is advantageous in workability compared with the one of condensation type, heat-curable type, and addition reaction type using a platinum catalyst on account of its ability to cure in a short time upon UV irradiation. However, the resulting product (silicone rubber elastomer) obtained in this manner is poor in tensile strength and storage stability, which limits its applications.
Another technology in the past is concerned with the use of a complex compound composed of titanium-(meth)acrylate-trialkoxide and β-diketone for micropatterning of an organic-inorganic hybrid thin film of photosensitive sol-gel system which is formed from polyfunctional silsesquioxane of double-decker type, as disclosed in Chemistry Letters Vol. 35, No. 10 (2006), pp. 1130-1131 (Non-Patent Document 1).
It is disclosed that the complex compound functions as an adhesion promoter that improves adhesion to the coating film when incorporated into the high-solid acrylic resin composition (see JP-A 2001-240620 (Patent Document 6)). This disclosure, however, suggests nothing about its application as a curing catalyst.
Citation List
Patent Document 1: JP-B S52-40334
Patent Document 2: JP-A S60-104158
Patent Document 3: JP-B S53-36515
Patent Document 4: JP-A S60-215009
Patent Document 5: JP 2647285
Patent Document 6: JP-A 2001-240620
Non-Patent Document 1: Chemistry Letters Vol. 35, No. 10 (2006), pp. 1130-1131